I'm very shortly about to order a significant amount of 009 track and
points - all PECO, I know that you can do better if you build your own but
I simply donít have the time.

One thing concerns me, I currently use SEEP point motors for my 00 gauge
track, all well and good and Iíve had no problems. However the "auxiliary
switch" so to speak is nothing more than a slider between two contacts on
the PCB and Iím concerned that if used with 009 gauge track there wonít be
enough throw on the points for the auxiliary to work as designed.

I don't think you'll have a problem Chris, but if you do you can always add
a small microswitch. You're aware, I assume, that the 009 points are
'Insulfrog' and that there's no need for a switch to change the frog
polarity.

Not in recent years - Peco 009 points have been electrofrog for a
looooong time
now, in fact the old insulfrog ones are very difficult to find any
more. I can't comment
on the effect of the limited throw on the auxiliary except to say that
it's probably
equivalent to N-gauge points, which will have the same throw.
Personally I use an arrangement of two capacitors and one side of a
DPDT switch which can operate a
twin solenoid without the need for passing contacts, and that gives me
the other side
of the switch spare for switching the frog.

Not in recent years - Peco 009 points have been electrofrog for a
looooong time
now, in fact the old insulfrog ones are very difficult to find any
more. I can't comment
on the effect of the limited throw on the auxiliary except to say that
it's probably
equivalent to N-gauge points, which will have the same throw.
Personally I use an arrangement of two capacitors and one side of a
DPDT switch which can operate a
twin solenoid without the need for passing contacts, and that gives me
the other side
of the switch spare for switching the frog.

I got a batch of 2200 microfarad electrolytics a while ago from Radio
Spares. One or two across the supply, then one for each point between
the switch centre contact and the negative power terminal. One solenoid
connects between the positive power terminal and one of the switch
contacts, and the other between the negative power terminal and the
other switch contact. There's never a DC circuit so there's no risk of
heating the solenoids; when it's switched one way it charges the
smoothing capacitor(s), the other way it charges the capacitor on the
point, and the only current flowing is the transient on switching as
the relevant capacitor discharges, so no need for a CDU. The drawback
is that you need a capacitor for every point motor (although you can
drive eg a crossover with a single capacitor) so it costs a bit.

The scheme wasn't my idea - I got it from a fellow Ipswich club member,
Bob Wilkinson, who's used it on several other layouts - but I wouldn't
use any other arrangement now.

On a related point, my daughter told me about a friend of hers who's
making a railgun using capacitors from the flash units from used
disposable cameras - apparently camera shops are happy to give them
away. Anybody know what sort of size capacitors they use?

Yes, I was being rather imprecise there. What I mean is that there's no
need for the constant current charging circuit that would be included
in a commercial CDU for electric pencil switching - an advantage for
me, as my electronics skills aren't up to much more than putting a few
capacitors and switches in the right place. The big advantage, as the
web page Martin gave says, is that the switch indicates the point
direction, so it works really well on a track diagram style control
panel.

Far be it for me to criticise a ciruit that has apparently been around
and in use since the 1950s, but the circuit Martin linked to only
discharges through one of the coils, and even then only if it has had a
chance to charge in the first place (hence the need to switch in each
direction on first power-up). The other coil will never have the
capacitor discharge through it - it can only be used to charge the cap.
Therefore the only initial "oomph" it will get is from what the power
supply can give it, rather than the capacitor.

That's why I put a pair of 2200 microfarad smoothing capacitors across
the supply - the other coil gets its impulse from the smoothing
capacitors discharging into the point capacitor. In fact, the circuit I
use is slightly different in that the capacitor and switch are the
other way round - the centre switch contact goes to the high voltage
rail and the two outer contacts go to the two solenoids. This means
that there are four wires per point motor rather than three, because
there can't be a common return; not much of an issue if you haven't got
too many turnouts, but on a big layout it'll be significant. I'm not
entirely sure the circuit in the diagram is right, but the version I
use works very well.

The cautionary note about the capacitors retaining their charge is
worth noting. After switching the power off I can switch a point back
and forth typically 10-12 times on one of my layouts, and waiting a few
hours doesn't make much difference. Only having a single 2200
microfarad capacitor across the supply seems to make it more like 3-4
times. Worth knowing if you're testing the circuit or altering the
wiring - those big capacitors can give quite a spark.

> Far be it for me to criticise a ciruit that has
> apparently been around and in use since
> the 1950s,

Why not you? <g> Nothing's perfect, criticise away. But it's
almost certainly older than that, the date I gave was the first
time I saw it. In those days the capacitors were massive, but
you could apparently buy them in your local ironmongers!

> The other coil will never have the capacitor discharge
> through it - it can only be used to charge the cap.
> Therefore the only initial "oomph" it will get is from
> what the power supply can give it, rather than the
> capacitor.

That's true, and why I didn't describe it as a "CDU", although
I see someone else tried to. This design needs an adequate
power supply. It doesn't make up for a feeble one.

In fact a power supply which is only barely adequate without
this circuit probably won't work with it -- you need a decent
sized transformer. I covered this point in the discussion on the
7mm Yahoo group (the reason I originally prepared the diagram).

The main advantages of this circuit are:

a) you can use any simple 2-way switch: toggle, slide, rotary,
home-made, whatever -- it doesn't require a momentary contact.

b) that means that you can also use it with relays and logic
to control the points.

c) most such switches usefully indicate the points direction
on the panel diagram. Especially if you use a rotary switch
with a pointer knob (but make sure it is a break-before-make
pattern).

d) it is impossible to burn out the point motor coils. (Unless the
capacitor fails -- make sure the capacitor voltage rating is at
least 25 volts for rectified 16vac supply.)

But a CDU it ain't. Or at least, only in one direction. If the points
are stiffer one way than the other, it's worth arranging that to be
the discharge direction.

p.s. The circuit actually described by Dave requires a 4-terminal
point motor (i.e. independent coils). The circuit at:
http://www.85a.ision.co.uk/info/solenoid_motor.htm
works the same way, but allows for 3-terminal point motors with
a common coil connection.

There's a circuit on the Modratec website (modratec.com/mud/php) that uses
diodes as well as a capacitor so that you only need one wire plus the return
wire to each point motor, instead of two. However, as far as I can see it is
still only a "half CDU" in the same way as Martin's and Dave's.

Has anyone come across a simple circuit that can operate both coils by
capacitor discharge using a toggle switch? I have an idea that with diodes
this should be possible -presumably you would need two capacitors per point
and a DPDT switch not an SPDT - but the details are beyond me unfortunately.

I'm interested in this as I have a bank of old Post Office keys that I want
to use as a lever frame but using capacitor discharge if possible.

> Has anyone come across a simple circuit that can operate
> both coils by capacitor discharge using a toggle switch?
> I have an idea that with diodes this should be possible

I would advise against mixing diodes with capacitor discharge.
For reliability you would need some robust high-current diodes
which will cost more than any saving in connecting wire.

Why bother with capacitor discharge units? All you need is to do
as Dave has done and beef up the power supply with some
capacitors across the output. This works fine for all methods
of working impulse solenoids -- passing contact switches,
centre-biased toggle switches, electric pencil + studs, etc.

If you add an additional capacitor per points-motor as in the
circuit we have been discussing, you don't even need a
momentary contact -- ordinary 2-way switches work too,
such as your PO keys.

I have no particular desire to make things more complicated than I need to;
in fact, quite the reverse. I suppose I have become used to CD point
operation over the years and for sure the three circuits I have seen -
yours, Dave's (if I have visualised it right) and the Modratec one - all
feature CD for one of the coils but not the other. As a result I had
wondered if it was possible to use CD for both coils, but I think I see what
you mean when you say that beefing up the power supply does away with the
need for this.

So if I can follow this train of thought a little further, bearing in mind
that words are no substitute for a diagram, I assume that your circuit also
could have a capacitor added in parallel with the power supply to give the
same effect as on Dave's? If so I think the Modratec circuit could as well.
One advantage of the Modratec circuit is that only two wires - one feed and
one return - need to be run from the switch to the point motor, thus cutting
down on the inter-baseboard connections. However, it does need diodes to
achieve this.

So, I think my choice will be between the Modratec circuit with added
capacitor (two wires, with diodes) and your circuit with added capacitor
(three wires, no diodes).

> I assume that your circuit also could have a capacitor
> added in parallel with the power supply to give the
> same effect as on Dave's?

Yes, sure. Whether you really need it depends on the size of
the power supply. I would use a 100VA transformer and a 10A
bridge rectifier, which is more than adequate by itself to throw
any ordinary points-motor. If you have a smaller supply you can
beef it up by adding one or more capacitors in parallel with the
rectified output. They should ideally be rated at least 35volts and
designed for high-ripple currents, but it depends how rugged a
system you want to build. Make sure the supply's bridge rectifier
is good for at least 50volts PIV, preferably 100volts.

> One advantage of the Modratec circuit is that only two wires -
> one feed and one return - need to be run from the switch to
> the point motor

The circuit at:

http://www.85a.ision.co.uk/info/solenoid_motor.htm

also requires only two wires per points-motor. The third connection
is to the layout ground (common return) which already exists on
each baseboard. If you prefer a separate return for the points-motors,
that is still only one additional wire per baseboard. There's no real
need, providing the existing return is done with good thick wire, as
it should be.